Technical Field
The present invention relates to a switching power source device, and particularly relates to a switching power source device that is a current-resonant DC-DC converter.
Background Art
Current resonance DC-DC converter switching power source devices can be made highly efficient and thin, and are therefore widely employed in liquid-crystal televisions, AC-DC adapters, and so on. In particular, current-resonant DC-DC converter switching power source devices that aim to reduce power consumed when an electronic device is not in use are being developed to comply with recent initiatives against global warming. Such a switching power source device has a standby mode for suppressing power consumption.
To further reduce the power consumed by the switching power source device when operating in standby mode, Patent Document 1 proposes a current-resonant switching power source device configured to carry out burst operation (a repeating intermittent oscillation operation) in which switching is carried out for a set period and then stopped for the next set period. Burst operation provides a period in which switching is paused, making it possible to greatly reduce the average standby power when the switching power source device is in standby mode. The switching power source device according to Patent Document 1 is furthermore configured to carry out a soft start operation when starting switching during burst operation in standby mode. Here, soft start operation refers to a switching frequency gradually decreasing over time after switching starts and ultimately converging on a given set value. Through the soft start, the resonant current of a resonant circuit gradually increases and the energy supplied from the primary side to the secondary side of a transformer gradually increases as well.
The switching power source device according to Patent Document 1 carries out a soft start when switching starts in the switching operations during burst operation; however, when the switching is stopped, the switching is stopped immediately in a state where the switching frequency is low and a high amount of energy is being supplied from the primary side to the secondary side, which makes it easy for noise to arise. In normal operation outside of the standby mode, the switching frequency does not drop to a frequency band audible to humans, and noise is not emitted from the transformer; however, when switching is stopped while in standby mode, noise corresponding to the resonant current at the stop of switching operations is emitted. In other words, a transient phenomenon arises, where the resonant current drops suddenly when the switching stops suddenly, and this transient phenomenon results in audible noise in the frequency component of the current flowing in the resonant circuit. The greater the resonant current is when the switching operations are stopped, the greater the audible noise frequency component is, resulting in more noise.
To prevent noise, Patent Document 2 proposes, in a switching power source device that performs fixed switching frequency PWM control, carrying out both a soft start and a soft end in the switching period during standby mode burst operation. Here, the soft end operation is an operation that gradually reduces an ON time ratio, which is a ratio of the ON period of a switching element to the switching cycle, as time passes, and reduces the energy sent from the primary side to the transformer. Noise occurring when switching is stopped in burst mode can be prevented by a soft end.
As indicated in FIG. 6 of Patent Document 2, a charge-discharge circuit 601d is provided on an output side of a voltage VEN for PWM pulse width control, and a long-period triangular wave is produced by charging/discharging a capacitor CSE connected to the charge-discharge circuit 601d with a constant current. In the soft start and soft end, the ON time ratio is gradually increased or reduced by comparing the triangular wave to a carrier signal outputted from an oscillator 6c. Furthermore, Patent Document 2 discusses achieving the same effects by applying a control circuit 6 that carries out such burst operation in a resonance-type power source.